Tag: Arecibo

Near-Earth asteroid twice as big as previously thought

By Phil Plait | June 22, 2012 11:00 am

On June 14, 2012, the asteroid 2012 LZ1 passed the Earth. It missed us by a wide margin, over 5 million kilometers (3 million miles), so there was no danger of impact. While it does get near us every now and again, using current orbital measurements we know we’re safe from an impact by this particular rock for at least 750 years. Phew.

Good thing, too. New observations using the Arecibo radio telescope in Puerto Rico indicate LZ1 is bigger than we first thought. Much bigger: it’s about a kilometer across, when it was thought to be half that size before these observations.

That’s a big difference. The problem is that the size of an asteroid is hard to determine. Even a big one may only appear as a dot in a telescope, so even though we may know its distance and trajectory very accurately, directly measuring its size isn’t possible. Usually, the size is estimated by knowing its distance and how bright it appears. In general, a bigger rock will look brighter than a smaller one at a given distance.

But that assumes they both reflect the same amount of light. Most asteroids reflect about 4% of the sunlight they receive (this property is called the albedo), but that depends on their surface. Some have darker surfaces, some brighter. If you don’t know how reflective it is, the size can only be estimated.

But the Arecibo telescope can actually directly measure the size of a nearby asteroid. It can send pulses of radio waves at an asteroid and then receive the reflected waves, much like a cop on the side of the road uses radar to measure a car’s speed. The method is technical (Emily Lakdawalla has a great explanation on her blog), but it was used for LZ1 to get the new size measurement. The picture above is the actual image generated using Arecibo when the rock was still 10 million km (6 million miles) from Earth. Apparently, LZ1 is much less reflective than assumed earlier, which is why the size was underestimated by a factor of two.

An asteroid this size hitting the Earth would be, um, bad. That’s big enough to be considered a global hazard, causing immense devastation. It might not be an extinction event — the dinosaur-killing asteroid was 10 km across, so it had 1000 times the mass of LZ1 — but it wouldn’t be fun. So I’m glad we’re safe from this guy for some time!

But I’ll be honest: LZ1 was only discovered a few weeks before it passed us. Asteroids this size passing near us are pretty rare (we haven’t had an impact from something this big for many, many millennia) so as usual I’m not panicking about this. But it just shows once again that we need more eyes on the sky, more people looking. And we need a plan in place in case we do see one with our name on it.

Related Posts:

Asteroid 2011 AG5: a football-stadium-sized rock to watch carefully
My asteroid impact talk is now on TED
Another tiny rock will pass Earth tomorrow
Updated movie of asteroid YU55, plus bonus SCIENCE
Just to be clear: asteroid YU55 is no danger to Earth
Armageddon delayed by at least a century… this time

Arecibo gives comet some radar love

By Phil Plait | October 29, 2010 10:42 am

Using a giant radio telescope like a cop’s radar gun, astronomers have made some pretty cool images of the nucleus of the comet Hartley 2:


Hartley 2 is a comet that is currently very close to the Earth as these things go: last week it passed us at a distance of about 18 million km (11 million miles). Astronomers took advantage of the close pass to ping the comet with radar pulses. By timing exactly how long it took the pulses to go from the telescope to the comet and back to Earth, they can create a map of the comet’s shape and other characteristics — something like how dolphins and bats use echolocation to map their surroundings… though, as Emily Lakdawalla at The Planetary Society Blog explains, it’s a bit more complicated.

From the images, it looks like the nucleus — the solid, central part of a comet — is highly elongated, about 2.2 km (1.4 miles) in length, and rotates once every 18 hours. We’ve only seen a handful of comets up close, and in general the nuclei are potato-shaped, so this one fits that description. The image has a scale of about 75 meters per pixel.

These observations were made to help out the EPOXI space mission, which will pass just 700 km (420 miles) from the nucleus of Hartley 2 on November 4. That means we’ll be getting some really cool close-up images and data from the comet very soon! Stay Tuned.

MORE ABOUT: Arecibo, comets, Hartley 2, radar

Armageddon delayed by at least a century… this time

By Phil Plait | April 29, 2010 10:51 am

What does a one-in-ten-million chance of apocalypse look like? Well, it used to look like this:


That is asteroid 2005 YU55, a near-Earth object (or NEO) that also happens to be a PHA, or potentially hazardous asteroid. It has an orbit which intersects the Earth, which means that someday it could possibly hit us.

Now before you panic — and I’ll make this clear: DON’T PANIC — that doesn’t mean you’ll wake up tomorrow to see flaming death streaking across the sky. Think of it this way: when you walk to the local convenience store to get a squishy, you have to cross the street. The path you take intersects the street, but as long as you don’t try to occupy the same spot as a moving car, you won’t get hit. Same with PHAs: their orbits cross the Earth’s orbit, but space is big. As long as the Earth and the asteroid aren’t at the same place at the same time, we’re OK.
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MORE ABOUT: 2005YU55, Arecibo

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